Zero-Heat-Flux, Deep Tissue Temperature Measurement Devices with Thermal Sensor Calibration

ABSTRACT

A zero-heat-flux DTT measurement device is constituted of a flexible substrate supporting an electrical circuit including a heater trace defining a heater, thermal sensors, and a thermal sensor calibration circuit.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/798,668, filed Apr. 7, 2010.

RELATED APPLICATIONS

This application contains material related to the following US patentapplications:

-   U.S. patent application Ser. No. 12/584,108, filed Aug. 31, 2009,    now U.S. Pat. No. 8,226,294; and,-   U.S. patent application Ser. No. 12/798,670, filed Apr. 7, 2012.

BACKGROUND

The subject matter relates to a device for use in the estimation of deeptissue temperature (DTT) as an indication of the core body temperatureof humans or animals. More particularly, the subject matter relates toconstructions of zero-heat-flux DTT measurement devices with provisionfor thermal sensor calibration.

Deep tissue temperature measurement is the measurement of thetemperature of organs that occupy cavities of human and animal bodies(core body temperature). DTT measurement is desirable for many reasons.For example, maintenance of core body temperature in a normothermicrange during the perioperative cycle has been shown to reduce theincidence of surgical site infection; and so it is beneficial to monitora patient's body core temperature before, during, and after surgery. Ofcourse noninvasive measurement is highly desirable, for the safety andthe comfort of a patient, and for the convenience of the clinician.Thus, it is most advantageous to obtain a noninvasive DTT measurement byway of a device placed on the skin.

Noninvasive measurement of DTT by means of a zero-heat-flux device wasdescribed by Fox and Solman in 1971 (Fox R H, Solman A J. A newtechnique for monitoring the deep body temperature in man from theintact skin surface. J. Physiol. January 1971:212(2): pp 8-10). TheFox/Solman system, illustrated in FIG. 1, estimates core bodytemperature using a temperature measurement device 10 with a controlledheater of essentially planar construction that stops or blocks heat flowthrough a portion of the skin. Because the measurement depends on theabsence of heat flux through the skin area where measurement takesplace, the technique is referred to as a “zero heat flux” (ZHF)measurement. Togawa improved the Fox/Solman technique with a DTTmeasurement device structure that accounted for multidimensional heatflow in tissue. (Togawa T. Non-Invasive Deep Body TemperatureMeasurement. In: Rolfe P (ed) Non-Invasive Physiological Measurements.Vol. 1. 1979. Academic Press, London, pp. 261-277). The Togawa device,illustrated in FIG. 2, encloses Fox and Solman's ZHF design in a thickaluminum housing with a cylindrical annulus construction that reduces oreliminates radial heat flow from the center to the periphery of thedevice.

The Fox/Solman and Togawa devices utilize heat flux normal to the bodyto control the operation of a heater that blocks heat flow from the skinthrough a thermal resistance in order to achieve a desired ZHFcondition. This results in a construction that stacks the heater,thermal resistance, and thermal sensors of a ZHF temperature measurementdevice, which can result in a substantial vertical profile. The thermalmass added by Togawa's cover improves the stability of the Fox/Solmandesign and makes the measurement of deep tissue temperature moreaccurate. In this regard, since the goal is zero heat flux through thedevice, the more thermal resistance the better. However, the additionalthermal resistance adds mass and size, and also increases the timerequired to reach a stable temperature.

The size, mass, and cost of the Fox/Solman and Togawa devices do notpromote disposability. Consequently, they must be sanitized after use,which exposes them to wear and tear and undetectable damage. The devicesmust also be stored for reuse. As a result, use of these devices raisesthe costs associated with zero-heat-flux DTT measurement and can pose asignificant risk of cross contamination between patients. It is thusdesirable to reduce the size and mass of a zero-heat-flux DTTmeasurement device, without compromising its performance, in order topromote disposability after a single use.

An inexpensive, disposable, zero-heat-flux DTT measurement device isdescribed and claimed in the priority application and illustrated inFIGS. 3 and 4. The device is constituted of a flexible substrate and anelectrical circuit disposed on a surface of the flexible substrate. Theelectrical circuit includes an essentially planar heater which isdefined by an electrically conductive copper trace and which surroundsan unheated zone of the surface, a first thermal sensor disposed in thezone, a second thermal sensor disposed outside of the heater trace, aplurality of electrical pads disposed outside of the heater trace, and aplurality of conductive traces connecting the first and second thermalsensors and the heater trace with the plurality of electrical pads.Sections of the flexible substrate are folded together to place thefirst and second thermal sensors in proximity to each other. A layer ofinsulation disposed between the sections separates the first and secondthermal sensors. The device is oriented for operation so as to positionthe heater and the first thermal sensor on one side of the layer ofinsulation and the second thermal sensor on the other and in closeproximity to an area of skin where a measurement is to be taken. As seenin FIG. 4, the layout of the electrical circuit on a surface of theflexible substrate provides a low-profile, zero-heat-flux DTTmeasurement device that is essentially planar, even when the sectionsare folded together.

Design and manufacturing choices made with respect to a zero-heat-fluxDTT measurement device can influence the operation of the device. Onesuch design choice relates to the thermal sensors used in the detectionof the zero-heat-flux condition. Given the importance of core bodytemperature, it is very desirable that the thermal sensors produceaccurate temperature data in order to enable reliable detection of thezero-heat-flux condition and accurate estimation of core bodytemperature. The tradeoff is between accuracy and cost of the thermalsensor. A number of thermal sensor devices are candidates for use inzero-heat-flux DTT measurement. Such devices include PN junctions,thermocouples, resistive temperature devices, and thermistors, forexample. Thermistors are a good choice for reasons of small size,handling convenience, ease of use, and reliability in the temperaturerange of interest. Their relatively low cost makes them desirablecandidates for single-use, disposable temperature measurement devices.

The magnitude of a thermistor's resistance changes in response to achange of the temperature of the thermistor. Thus, to determine themagnitude of the temperature, the thermistor's resistance is measuredand converted to a temperature value using a known relationship.However, batch-to-batch manufacturing variances can yield a large rangevariance in thermistor resistance. For example, low-cost thermistors canexhibit a range of ±5% in resistance values from device to device at agiven temperature, which yields a range of ±2.5° C. in temperature. Sucha large range in variance can compromise the accuracy and reliability ofzero-heat-flux temperature measurement. Thus, while it is desirable touse such thermistors in order to limit the cost of parts and labor inmanufacturing zero-heat-flux DTT measurement devices, it is important toreduce, if not remove, the effects of resistance variance on deviceoperation.

The range of thermistor resistance variance can be neutralized bycalibration of thermistor resistance using known methods, such as theSteinhart-Hart equation, which require knowledge of coefficients derivedfrom values of thermistor resistance measured at fixed temperatures.When a thermistor is operated, the coefficients are used in knownformulas to correct or adjust the magnitude of its indicated resistance.Such correction is called calibration.

Preferably, once determined, the coefficients are stored in a memorydevice so as to be available for use when the thermistor is operated.For example, as described in Japanese patent publication 2002-202205, adeep temperature measuring device includes a temperature probeconstructed for zero-heat-flux measurement and a cable projecting fromthe probe. One end of the cable terminates on the probe, and theopposite end in a connector. Signal wires run in the cable between theprobe and the connector. A read-only memory (ROM) is mounted in theconnector casing, away from the probe. Information stored in the ROMincludes probe classification and thermistor coefficients. Since thethermistor coefficients are unique to the thermistors on the probe, theROM must be permanently associated with the probe, and so the cable ispermanently fixed to the probe. The connector detachably plugs into atemperature measurement system. At start-up, the system reads theclassification and coefficient information from the ROM. The system usesthe coefficient information to calibrate thermistor readings obtainedfrom the probe, thereby to reduce or remove the effects of resistancevariation from the zero-heat-flux process.

The cable of the deep temperature measuring device with its permanentconnector results in a complex construction that is costly tomanufacture, difficult to store, and awkward to handle. A fullcomplement of probes for a temperature measuring system has as manycables as probes. The probes are reusable, and so the problems describedabove in connection with the Fox/Solman and Togawa devices arecompounded by the presence of the cables.

SUMMARY

An object of an invention completed in respect of the problems describedabove is to provide a zero-heat-flux DTT measurement device constitutedof a flexible substrate and a zero-heat-flux electrical circuit disposedon a surface of the flexible substrate with thermal sensor calibrationcoefficients provided from a circuit mounted on the substrate.

Another object of an invention completed in respect of the problemsdescribed above is to eliminate a cable and connector as integral partsof a zero-heat-flux DTT probe without sacrificing the cost-savingbenefits of inexpensive thermal sensors.

Another object of an invention completed in respect of the problemsdescribed above is to provide thermal sensor calibration for azero-heat-flux DTT measurement device constituted of a flexiblesubstrate and electrically conductive traces on a surface of thesubstrate for a heater and at least two thermal sensors.

These and other objects are achieved with a zero-heat-flux DTTmeasurement device constituted of a flexible substrate supporting anelectrical circuit including a heater trace defining a heater, thermalsensors, and a thermal sensor calibration circuit.

Preferably, the thermal sensor calibration circuit includes aprogrammable memory storing thermal measurement information includingthermal sensor calibration coefficients.

These and other objects are achieved with a zero-heat-flux DTTmeasurement device constituted of a flexible substrate including acenter section, a tab extending from the periphery of the centersection, and a tail extending from the periphery of the center section,and an electrical circuit on a surface of the flexible substrate, theelectrical circuit including a heater trace defining a heatersurrounding a zone of the surface, a first thermal sensor disposed inthe zone, a second thermal sensor disposed on the tail, a memory devicedisposed on the substrate outside of the heater trace, a plurality ofelectrical pads disposed on the tab, and a plurality of conductivetraces connecting the first and second thermal sensors, the memorydevice, and the heater trace with the plurality of electrical pads.

Preferably, the memory device includes a multi-pin memory device storingthermal measurement information including thermal sensor calibrationcoefficients

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a first prior art deep tissuetemperature measurement system including a ZHF DTT measurement device.

FIG. 2 is a schematic side sectional diagram of a second prior art deeptissue temperature measurement system including a ZHF deep tissuetemperature measurement device with an aluminum cap.

FIG. 3 is a plan view of a side of a flexible substrate showing anelectrical circuit disposed on a surface of the substrate fortemperature measurement.

FIG. 4 is a side sectional view of a temperature device thatincorporates the electrical circuit of FIG. 3.

FIG. 5 is an exploded assembly view, in perspective, showing elements ofthe temperature device of FIG. 4.

FIGS. 6A-6F illustrate a method of temperature device manufacture basedon the temperature device of FIGS. 4 and 5.

FIG. 7A is a first side sectional, partly schematic illustration of azero-heat-flux DTT measurement device illustrating components of amulti-layer construction.

FIG. 7B is a second side sectional, partly schematic illustration of thezero-heat-flux DTT measurement device of FIG. 7A rotated to illustrate athermal sensor calibration circuit included in the multi-layerconstruction.

FIG. 8A illustrates a first construction of the zero-heat-flux DTTmeasurement device construction of FIG. 7, and FIG. 8B is a schematicdiagram including elements of the measurement device.

FIG. 9 is a block diagram illustrating a temperature measurement system.

FIG. 10 illustrates a second construction of the zero-heat-flux DUmeasurement device construction of FIG. 7.

FIG. 11 illustrates a third construction of the zero-heat-flux DTTmeasurement device construction of FIG. 7.

FIG. 12 illustrates a fourth construction of the zero-heat-flux DTTmeasurement device construction of FIG. 7.

FIG. 13 illustrates a fifth construction of the zero-heat-flux DTTmeasurement device construction of FIG. 7.

FIG. 14 illustrates a sixth construction of the zero-heat-flux DTTmeasurement device construction of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is desirable that zero heat flux, deep tissue temperature measurementdevice constructions include on-board thermal sensor calibrationinformation in order to remove the effects of thermal sensor variance ondevice operation.

A temperature device for zero-heat-flux DTT measurement includes aflexible substrate with at least two thermal sensors disposed in aspaced-apart relationship and separated by one or more flexible layersof thermally insulating material. Preferably the sensors are maintainedin a spaced apart relationship by a flexible thermal (and electrical)insulator. The substrate supports at least the thermal sensors, theseparating thermal insulator, a thermal sensor calibration circuit, anda heater.

Although temperature device constructions are described in terms ofpreferred embodiments comprising representative elements, theembodiments are merely illustrative. It is possible that otherembodiments will include more elements, or fewer, than described. It isalso possible that some of the described elements will be deleted,and/or other elements that are not described will be added. Further,elements may be combined with other elements, and/or partitioned intoadditional elements.

A Zero-Heat-Flux DTT Measurement Device

A layout for a zero-heat-flux, DTT measurement device is illustrated inFIG. 3. The device includes an electrical circuit disposed on a flexiblesubstrate in order to adapt or conform the physical configuration of thetemperature measurement device to differing contours encountered atdifferent temperature measurement locations. Preferably, but notnecessarily, the flexible substrate is constructed or fabricated to havea plurality of contiguous sections. For example, the flexible substrate100 has three contiguous sections 102, 104, and 106. The first, orcenter, section 102 is substantially circular in shape. The secondsection (or “tail”) 104 has the shape of a narrow, elongate rectanglethat extends in a first radial direction from the periphery of the firstsection 102. Where the center section and the tail join at 105, theperiphery of the center section has a straight portion and the width ofthe tail is reduced. The third, or tab, section 106 has the shape of abroad, elongate rectangle that extends in a second radial direction fromthe periphery of the center section 102. Preferably, the tail and tabare aligned along a diameter of the center section.

As per FIG. 3, the elements of the electronic circuit are disposed on asingle surface, on a first side 108 of the flexible substrate. A firstthermal sensor 120 is positioned inside the outer perimeter of thecenter section 102, preferably near or at the center of the centersection 102. An electrically conductive heater trace 122 defines aheater with a shape that surrounds or encircles a zone 121 in which thefirst thermal sensor 120 is located. In the preferred embodimentillustrated in FIG. 3, the heater trace has an annular shape thatincludes a circular array of wedge-shaped heater zones 124 that surroundor encircle the zone 121 and the first thermal sensor 120 which isdisposed in the zone. A second thermal sensor 126 is positioned on thetail 104. A plurality of electrical connection pads 130 is located onthe tab 106. The heater trace includes two electrically conductive tracesections that terminate in the connection pads 130 a and 130 b. Twoelectrically conductive traces extend between mounting pads on which thefirst thermal sensor 120 is mounted and the connection pads 130 c and130 d. Two additional electrically conductive traces extend betweenmounting pads on which the second thermal sensor 126 is mounted and theconnection pads 130 e and 130 f.

In the specific layout shown of the preferred embodiment shown in FIG.3, the path of the heater trace 122 crosses the paths of the two tracesfor the second thermal sensor 126. In this case, the continuity of theheater trace is preferably, but not necessarily, maintained by anelectrically conductive zero-ohm jumper 132 which crosses, and iselectrically isolated from, the two traces for the second thermal sensor126. In other embodiments, the continuity of the heater trace 122 canalso be maintained by vias to the second side of the flexible substrate,by running the thermal sensor traces around the periphery of the firstside of the flexible substrate, by a jumper wire instead of the zero-ohmresistor, or by any equivalent solution.

The flexibility or conformability of the flexible substrate can beenhanced by a plurality of slits 133 that define zones which move orflex independently of each other. In the preferred embodiment, the slits133 are made in the center section 102 in a pattern that follows oraccommodates the layout of the heater trace 122. The pattern at leastpartially separates the heater zones 124 so as to allow any one of theheater zones 124 to move independently of any other heater zone. Thepreferred pattern of slits is a radial pattern in that each slit is madealong a respective radius of the circular center section 102, betweenadjacent heater zones, and extends along the radius from the peripheryof the center section 102 toward the center of the circular shape of thesection. This is not meant to exclude other possible slit configurationsdetermined by the different shapes of the heater trace layout and theflexible substrate sections.

Sections of the flexible substrate are brought or folded together aboutan insulator to provide thermal resistance between the first and secondthermal sensors 120 and 126 in a configuration that is preferred for ZHFtemperature measurement. For example, at least the center and tailsections 102 and 104 of the flexible substrate are brought or foldedtogether about a flexible insulator. Preferably, the first and secondthermal sensors 120 and 126 are thereby disposed on respective sides ofa thermal insulator. In this regard, with reference to FIGS. 3 and 4,the center section 102 and tail 104 are folded together about a flexiblelayer of insulating material 140. The layer 140 provides thermal andelectrical resistance between the thermal sensors; it also supports thethermal sensors in a spaced-apart configuration.

A flexible temperature measurement device construction includes anelectrical circuit laid out on a side of a flexible substrate as shownin FIG. 3. With two sections of the flexible substrate brought or foldedtogether so as to sandwich a flexible insulator, the construction has amultilayer structure as best seen in FIG. 4. Thus, a temperaturemeasurement device 200 includes the electrical circuit laid out on thesurface of the first side 108 of the flexible substrate 100. The centraland tail sections 102 and 104 are brought or folded together about theflexible insulating layer 140 so as to provide a thermal resistancebetween the first and second thermal sensors 120 and 126. The flexibleinsulating layer also maintains the first and second thermal sensorsdisposed in a spaced relationship. Preferably, but not necessarily, thesecond thermal sensor 126 is aligned with the first thermal sensor on aline 202 which passes through the zone 121 that is surrounded by theheater trace (seen in FIG. 3). The temperature measurement devicefurther includes a flexible heater insulator 208 attached to a secondside 109 of the substrate 100, over the center section 102.

The layout of the electrical circuit illustrated in FIG. 3 locates allof the circuit components on a single surface on one side of theflexible substrate 100. This layout confers several advantages. First,it requires only a single fabrication sequence to lay down traces forthe heater, the thermal sensors, and the connection pads, therebysimplifying manufacture of the device. Second, when the sectionscarrying the thermal sensors are folded together, the thermal sensorsare maintained within a thermally and mechanically controlledenvironment.

Another benefit of the preferred layout shown in FIG. 3 is that thefirst thermal sensor 120 is physically separated from the heater, in azone 121 that is surrounded or encircled by the heater trace 122, andnot stacked under it as in the Fox/Solman system. When the temperaturemeasurement device is activated, the heater is turned on and the heatproduced thereby travels generally vertically from the heater to thepatient, but only medially to the first thermal sensor. As a result, thejump in temperature that occurs when the heater is activated is notimmediately sensed by the first thermal sensor, which improves controlof the heater and stability of the temperature measurement withoutrequiring an increase in thermal mass of the temperature measurementdevice. Thus, the first temperature sensor 120 is preferably located inthe same plane, or on the same surface, as the heater trace 122 (and caneven be elevated slightly above the heater trace), and substantially inor in alignment with the zone 121 of zero heat flux.

It is desirable that the temperature measurement device support apluggable interface for convenience and for modularity of a patientvital signs monitoring system. In this regard, and with reference toFIGS. 3 and 4, the tab 106 is configured with the array of pads 130 soas to be able to slide into and out of connection with a connector (notshown). In order to provide a physically robust structure capable ofmaintaining its shape while being connected and disconnected, the tab106 is optionally stiffened. In this regard, a flexible stiffener 204 isdisposed on the second side 109 of the flexible substrate 100. Thestiffener extends substantially coextensively with the tab 106 and atleast partially over the center section 102. As best seen in FIG. 4, thestiffener 204 is disposed between the second side 109 of the flexiblesubstrate 100 and the flexible insulator 208. A key to align the tab 106and prevent misconnection with an electrical connector (not shown) andto retain the connector on the tab may be provided on the device 200.For example, with reference to FIG. 5, such a key includes an opening209 through the stiffener and tab. In operation, the opening 209 wouldreceive and retain a retractable, spring-loaded pawl on the casing of aconnector.

The temperature measurement device 200 is mounted on a region of skinwhere temperature is to be measured with the second thermal sensor 126closest to the skin. As seen in FIG. 4, a layer of adhesive 222 isdisposed on the second side 109, on the layer of insulation 140 and theportion of the tail 104 where the second sensor 126 is located. Arelease liner (not shown in this figure) may be peeled from the layer ofadhesive 222 to prepare the device 200 for attachment to the skin. Whendeployed as shown in FIG. 4, a pluggable signal interface between theelectrical circuit on the device 200 and a temperature measurementsystem is provided through the plurality of electrical connection pads130 located in the tab 106. The signals transferred therethrough wouldinclude at least heater activation and thermal sensor signals.

Use of an electrical circuit on a flexible substrate greatly simplifiesthe construction of a disposable temperature device for estimating deeptissue temperature, and substantially reduces the time and cost ofmanufacturing such a device. In this regard, manufacture of atemperature measurement device incorporating an electrical circuit laidout on a side of the flexible substrate 100 with the circuit elementsillustrated in FIG. 3 may be understood with reference to FIGS. 5 and6A-6F. Although a manufacturing method is described in terms ofspecifically numbered steps, it is possible to vary the sequence of thesteps while achieving the same result. For various reasons, some of thesteps may include more operations, or fewer, than described. For thesame or additional reasons, some of the described steps may be deleted,and/or other steps that are not described may be added. Further, stepsmay be combined with other steps, and/or partitioned into additionalsteps.

In FIG. 6A, the traces and pads for an electrical circuit are fabricatedon a first side 108 of a flexible substrate 100 with a center section102, a tail 104 extending from the center section, and a tab 106extending from the center section. The electronic elements (first andsecond thermal sensors) are mounted to the traces to complete anelectrical circuit (which is omitted from these figures for convenience)including the elements of FIG. 3, laid out as shown in that figure. Ifused, the pattern of slits 133 separating the heater zones may be madein the center section in this manufacturing step.

As per FIG. 6B, in a second manufacturing step, a stiffener 204 islaminated to a second side of the flexible substrate. As best seen inFIG. 5, the stiffener has a portion shaped identically to the tab andnarrows to an elongated portion with a circular tip. When laminated tothe second side 109, the stiffener substantially extends over the taband partially over the center section, beneath the zone 121 where thefirst thermal sensor is located. Preferably, an adhesive film (notseen), or equivalent, attaches the stiffener to the second side of theflexible substrate.

As per FIG. 6C, in a third manufacturing step, a flexible layer 208 ofinsulating material is attached by adhesive or equivalent to the firstside of the flexible substrate, over substantially all of the centersection and at least a portion of the stiffener. This layer is providedto insulate the heater from the ambient environment. As best seen inFIG. 5, this flexible layer may include a truncated tab 210 thatprovides additional reinforcement to a pluggable connection between thetab 106 and a system connector.

As per FIG. 6D, in a fourth manufacturing step, a flexible central layerof insulating material 140 is attached to the first side 108, over thecenter section, to cover the heater trace and the first thermal sensor.As best seen in FIG. 5, this flexible layer may also include a truncatedtab 141 that provides additional reinforcement to a pluggable connectionbetween the tab and a system connector.

As per FIG. 6E, in a fifth manufacturing step, the tail 104 is foldedover the central layer of insulating material 140 such that the firstand second thermal sensors are maintained by the central layer in thepreferred spaced relationship.

As per FIG. 6F, in a sixth manufacturing step, a layer of adhesive (notseen) with a release liner 226 is attached to the central insulatinglayer, over the central insulating layer with the tail folded thereto.As best seen in FIG. 5, the release liner 226 may have a shape thatcorresponds to the central section 102 and tab 106.

In a best mode of practice, a temperature measurement device accordingto this specification has been fabricated using the materials and partslisted in the following table. An electrical circuit with copper tracesand pads conforming to FIG. 3 was formed on a flexible substrate ofpolyimide film by a conventional photo-etching technique and thermalsensors were mounted using a conventional surface mount technique. Thedimensions in the table are thicknesses, except that Ø signifiesdiameter. Of course, these materials and dimensions are onlyillustrative and in no way limit the scope of this specification. Forexample, traces may be made wholly or partly with electricallyconductive ink.

Table of Materials and Parts: I Element Material Representativedimensions Flexible substrate Kapton ® polyimide film with depositedSubstrate 100: 0.05 mm and photo-etched copper traces and pads Thermalsensors NTC thermistors, Part # R603-103F- 3435-C, Redfish SensorsFlexible insulating Closed cell polyethylene foam with Insulator 208:Ø50 × 1.5 mm layers skinned major surfaces coated with Insulator 140:Ø50 × 3.0 mm pressure sensitive adhesive (PSA) Stiffener Polyethyleneterephthalate (PET) Stiffener 204: 0.25 mmZero-Heat-Flux DTT Measurement Devices with Thermal Sensor Calibration

Zero-heat-flux DTT measurement devices according to FIG. 3 and thepreceding description have been fabricated, assembled, and clinicallytested. We have found that it is desirable to further adapt theconstruction of such devices by provision of thermal sensor calibrationcircuitry that enables reliable estimation of deep tissue temperaturemeasurement by zero-heat-flux operation. Desirably, the placement of thethermal sensor calibration circuitry on the measurement devices andprovision of a pluggable connector interface at the periphery of themeasurement device eliminate the need for a cable permanently fixed tothe measurement device.

These objectives are met by zero-heat-flux DTT measurement deviceconstructions with a flexible substrate that supports an electricalcircuit in which a heater trace is disposed on a first substrate layerto define a heater facing one side of a layer of thermally insulatingmaterial and surrounding a zone of the first substrate layer, a firstthermal sensor is disposed in the zone, a thermal sensor calibrationcircuit is disposed on the first substrate layer outside of the heater,a second thermal sensor is disposed on the second substrate layer, aplurality of electrical pads is disposed outside of the heater trace ona substrate surface, and a plurality of conductive traces connects theheater trace, the first and second thermal sensors and the thermalsensor calibration circuit with the plurality of electrical pads.

These objectives are also met by zero-heat-flux DTT measurement deviceconstructions with a flexible substrate that supports an electricalcircuit in which a heater trace is disposed on a first substrate layerto define a heater facing one side of a layer of thermally insulatingmaterial and surrounding a zone of the first substrate layer, a firstthermal sensor is disposed in the zone, a second thermal sensor isdisposed on the second substrate layer, and a plurality of electricalcontact pads is disposed outside of the heater trace on a substratesurface to provide an interface where a connector can be detachablycoupled to the measurement device. A memory device storing thermalsensor calibration information is disposed on the first substrate layer,and a plurality of conductive traces connects the heater trace, thefirst and second thermal sensors and the memory device with theplurality of electrical pads.

FIG. 7A is a sectional, partially-schematic illustration of a preferredzero-heat-flux DTT measurement device construction. FIG. 7B is asectional, partially-schematic illustration of the preferredzero-heat-flux DTT measurement device construction in which the sectionis rotated from the view of FIG. 7A. Not all elements of the measurementdevice are shown in these figures; however, the figures do showrelationships between components of the construction that are relevantto zero-heat-flux measurement with thermal sensor calibration. As perFIG. 7A, the measurement device 700 includes flexible substrate layers,a layer of thermally insulating material, and an electrical circuit. Theelectrical circuit includes a heater 726, a first thermal sensor 740,and a second thermal sensor 742. The heater 726 and the first thermalsensor 740 are disposed in or on a flexible substrate layer 703, and thesecond thermal sensor 742 is disposed in or on a flexible substratelayer 704. The first and second substrate layers 703 and 704 areseparated by a flexible layer 702 of thermally insulating material. Theflexible substrate layers 703 and 704 can be separate elements, but itis preferred that they be sections of a single flexible substrate foldedaround the layer of insulating material. Preferably, adhesive film (notshown) attaches the substrate to the insulating layer 702. A layer ofadhesive material 705 mounted to one side of the substrate layer 704 isprovided with a removable liner (not shown) to attach the measurementdevice to skin. Preferably, a flexible layer 709 of insulating materiallies over the layers 702, 703, and 704 and is attached by adhesive film(not shown) to one side of the substrate layer 702. The insulating layer709 extends over the heater 726 and the first thermal sensor 740.

As seen in FIG. 7B, the electrical circuit further includes a thermalsensor calibration circuit 770 and electrical pads 771 disposed in or onthe flexible substrate layer 703. The thermal sensor calibration circuit770 is positioned outside of the heater 726, preferably between theheater 726 and the electrical pads 771. The electrical pads 771 arepositioned on a section 708 of the substrate layer 703 that projectsbeyond the insulating layer 709 so as to be detachably coupled with aconnector 772 fixed to the end of a cable 787. As will be explained indetail with reference to other figures, the thermal calibration circuit770 includes a programmable memory storing thermal sensor calibrationand other information. Presuming that the thermal sensors 740 and 742are thermistors, the thermal sensor calibration information can includeone or more unique calibration coefficients for each thermistor.Location of the thermal sensor circuit on the measurement device 700,between the heater 726 and the electrical pads 771 permanentlyassociates the stored thermal sensor calibration information with themeasurement device 700. Thus, the need for a cable, with connector,permanently attached to the measurement device is eliminated. Moreover,since the cable 787 and connector 772 do not store unique calibrationinformation, they can be used for any zero-heat-flux DTT measurementdevice configured in accordance with FIGS. 7A and 7B. Finally, locationof the thermal sensor circuit 770, with stored thermal sensorcalibration information, on the measurement device 700, enables use oflow cost thermal sensors.

With reference to FIGS. 7A and 7B, the measurement device 700 isdisposed with the second thermal sensor 742 nearest the skin. The layer702 is sandwiched between the first and second substrate layers 703 and704 so as to separate the heater 726 and first thermal sensor 740 fromthe second thermal sensor 742. In operation, the layer 702 acts as alarge thermal resistance between the first and second thermal sensors,the second thermal sensor 742 senses the temperature of the skin, andthe first thermal sensor senses the temperature of the layer 702. Whilethe temperature sensed by the first thermal sensor 740 is less than thetemperature sensed by the second thermal sensor 742, the heater isoperated to reduce heat flow through the layer 702 and the skin. Whenthe temperature of the layer 702 equals that of the thermal sensor 742,heat flow through the layer 702 stops and the heater is switched off.This is the zero-heat-flux condition as it is sensed by the first andsecond sensors 740 and 742. When the zero-heat-flux condition occurs,the temperature of the skin, indicated by the second thermal sensor, isinterpreted as core body temperature. In some zero-heat-flux DTTmeasurement device constructions that are to be described in detail, theheater 726 can include a central heater portion 728 that operates with afirst power density, and a peripheral heater portion 729 surrounding thecentral heater portion that operates with a second power density higherthan the first power density. Of course, the flexibility of thesubstrate permits the measurement device 700, including the heater 726,to conform to body contours where measurement is made.

With reference to FIG. 8A, a first construction of a zero-heat-flux DTTmeasurement device 700 with thermal sensor calibration includes aflexible substrate 701. Preferably, but not necessarily, the flexiblesubstrate 701 has contiguous sections 705, 706, and 708. Preferably, butnot necessarily, the first, or center, section 705 is substantiallycircular in shape. The second section (or “tail”) 706 has the shape of anarrow, elongated rectangle with a bulbous end 707 that extendsoutwardly from the periphery of the center section 705 in a firstdirection. The third section (or “tab”) is the extended section 708 seenin FIG. 7B. The tab 708 has the shape of a wide rectangle that extendsoutwardly from the periphery of the center section 705 in a seconddirection. Opposing notches 710 are formed in the tab 708 to receive andretain respective spring-loaded retainers of a connector (such as theconnector 772 seen in FIG. 7B). Preferably but not necessarily, the tail706 is displaced from the tab 708 by an arcuate distance of less than180° in either a clockwise or a counterclockwise direction.

As per FIG. 8A, an electrical circuit 720 is disposed on the flexiblesubstrate 701. Preferably, but not necessarily, the elements of theelectrical circuit 720 are located on the surface 721 of the flexiblesubstrate 701. The electrical circuit 720 includes at least anelectrically conductive heater trace, thermal sensors, a thermal sensorcalibration circuit, electrically conductive connective trace portions,and electrical connection pads. The heater trace 724 defines a generallyannular heater 726 surrounding a zone 730 of the substrate 701 intowhich no portion of the heater trace 724 extends; in this regard, thezone 730 is not directly heated when the heater operates. The zone 730occupies a generally circular portion of the surface 721. Morecompletely, the zone 730 is a cylindrical section of the substrate 701which includes the portion of the surface 721 seen in FIG. 8A, thecounterpart portion of the opposing surface (not seen in this figure),and the solid portion therebetween. Preferably, but not necessarily, thezone 730 is centered in the center section 705 and is concentric withthe heater 726. The first thermal sensor 740 is mounted on mounting padsformed in the zone 730. The second thermal sensor 742 is mounted onmounting pads disposed outside of the generally annular heater 726;preferably, these mounting pads are formed generally near the end of thetail 706, for example, in or near the center of the bulbous end 707 ofthe tail. In some constructions the thermal sensor calibration circuit770 includes at least one multi-pin electronic circuit device mounted onthe measurement device 700. For example the thermal sensor calibrationcircuit 770 can be constituted of an electrically-erasable programmableread/write memory (EEPROM) mounted on mounting pads formed on a portionof the surface 721 on the center section 705 near or adjacent the tab708. The electrical connection pads (“electrical pads”) 771 are formedon the surface 721, in the tab 708. A plurality of conductive traceportions connects the first and second thermal sensors, the thermalsensor calibration circuit 770, and the heater trace 724 with aplurality of the electrical pads 771. Preferably, but not necessarily,at least one electrical pad 771 is shared by the thermal sensorcalibration circuit 770 and one of the heater 726, the first thermalsensor 740, and the second thermal sensor 742.

As seen in FIG. 8A, preferably, but not necessarily, the center section705 has formed therein a plurality of slits 751, 752 to enhance theflexibility and conformability of the flexible substrate. The slitsextend radially from the periphery toward the center of the centersection 705. The slits define zones which move or flex independently ofeach other. The layout of the heater trace 724 is adapted to accommodatethe slits. In this regard, the heater trace follows a zigzag orswitchback pattern with legs that increase in length from the peripheryof the zone 730 to the ends of the longer slits 751 and then, after astep decrease at those ends, generally increase in length again to theouter periphery of the heater 726 in the zones defined by the slits. Asillustrated, the construction of the heater has a generally annularshape centered in the zone 730, although the annularity is interruptedby the slits. Alternatively, the annular shape can be viewed asincluding a peripheral annulus of wedge-shaped heater zones surroundinga generally continuous central annulus.

Preferably, but not necessarily, the heater 726 has a non-uniform powerdensity heater structure that can be understood with reference to FIG.8A. In this construction, the heater 726 includes a central portion 728(indicated by lightly drawn lines) having a first power density and aperipheral portion 729 (indicated by heavily drawn lines) whichsurrounds the central portion 728 and has a second power density higherthan the first power density. The heater trace 724 is continuous andincludes two ends, a first of which transitions to electrical pad 5, andthe second to electrical pad 6. However, because of the slits, each ofthe central and peripheral portions 728 and 729 includes a plurality ofsections arranged in a sequence, in which the sections of the centralportion 728 alternate with the sections of the peripheral portion.Nevertheless, the annular structure of the heater arrays the sections ofthe central portion 728 generally in a central annulus around the zone730, and arrays the sections of the peripheral portion 729 around thecentral portion 728. When the heater 726 is operated, the centralportion 728 produces a central annulus of heat at the first powerdensity surrounding the zone 730 and the peripheral portion 729 producesa ring-shaped annulus of heat at the second power density that surroundsthe central annulus of heat.

Preferably the heater trace 724 is continuous, but exhibits a nonuniformpower density along its length such that the central heater portion 728has a first power density and the peripheral portion 729 has a secondpower density that is greater than the first power density. With thisconfiguration, a driving voltage applied to the heater 726 will causethe central heater portion 728 to produce less power per unit of heaterarea of the heater trace than the outer heater portion 729. The resultwill be a central annulus of heat at a first average power surrounded bya ring of heat a second average power higher than the first.

The differing power densities of the heater portions 728 and 729 may beinvariant within each portion, or they may vary. Variation of powerdensity may be step-wise or continuous. Power density is most simply andeconomically established by the width of the heater trace 724 and/or thepitch (distance) between the legs of a switchback pattern. For example,the resistance, and therefore the power generated by the heater trace,varies inversely with the width of the trace. For any resistance, thepower generated by the heater trace also varies inversely with the pitchof (distance between) the switchback legs.

The electrical circuit 720 on the flexible substrate 701 seen in FIG. 8Ais shown in schematic form in FIG. 8B. The electrical pads 771 on thetab 708 numbered 1-6 in FIG. 8A correspond to the identically-numberedelements in FIG. 8B. The number of electrical pads shown is merely forillustration. More, or fewer, electrical pads can be used; any specificnumber is determined by design choices including the specific deviceconfiguration of the thermal sensor calibration circuit, the heaterconstruction, the number of thermal sensors, and so on. In someconstructions it is desirable to utilize one or more of the electricalpads for electrical signal conduction to or from more than a singleelement of the electrical circuit 720 in order to minimize the number ofelectrical pads, thereby simplifying the circuit layout, minimizing thesize and mass of the tab 708, and reducing interface connector size.

Presume that the thermal sensor calibration circuit 770 includes amulti-pin electronically programmable memory (EEPROM) such as a24AA01T-I/OT manufactured by Microchip Technology and mounted bymounting pads to the zero-heat-flux DTT measurement device 700. FIGS. 8Aand 8B illustrate a construction in which one or more electrical padsare shared by at least two elements of the electrical circuit. In thisregard:

one lead of the second thermal sensor 742 and pin 1 of the thermalsensor calibration circuit 770 are connected by conductive traceportions to electrical pad 1;

leads of the first and second thermal sensors 740 and 742 and pin 4 ofthe thermal sensor calibration circuit 770 are connected by conductivetrace portions to electrical pad 2;

one lead of the first thermal sensor 740 and pin 3 of the thermal sensorcalibration circuit 770 are connected by conductive trace portions toelectrical pad 3;

pins 2 and 5 of the thermal sensor calibration circuit 770 are connectedby a conductive trace portion to electrical pad 4;

the return end of the heater trace 724 is connected by a conductivetrace portion to electrical pad 5; and

the input end of the heater trace 724 is connected by a conductive traceportion to electrical pad 6.

With reference to FIGS. 7A, 7B, and 8A, when the measurement device 700is assembled, the center section 705 and tail 706 are folded togetherabout a flexible layer of insulating material such as the layer 702. Thelayer 702 provides thermal resistance and electrical insulation betweenthe thermal sensors; it also supports the thermal sensors in aspaced-apart configuration. In other words, the first and second thermalsensors 740 and 742 are disposed on respective layers of substratematerial that are separated by the layer of insulating material with theheater and first thermal sensor facing one side of the layer ofinsulating material and the second thermal sensor facing the other.

The zero-heat-flux DTT measurement device 700, with the electricalcircuit 720 laid out on one or more sides of the flexible substrate 701as illustrated in FIG. 8A, can be manufactured and assembled in themanner illustrated in FIGS. 5 and 6A-6F, using materials identified inthe Table of Materials and Parts II. Preferably, the measurement deviceis constructed with a stiffener comprising a separate piece or a layerof material painted, deposited, or formed on the tab 708 and thenhardened. The stiffener reduces the flexibility of the tab 708, therebyenabling it to be reliably coupled to and decoupled from a connector.Preferably, with reference to FIGS. 4 and 8A, such a stiffener for thetab 708 (FIG. 8A) is disposed on the side of the flexible substrate 701that corresponds to the second side 109 of the flexible substrate 100(FIG. 4). The stiffener extends substantially coextensively with the tab708, and at least partially over the center section 705, but stops shortof the zone 730, approximately where indicated by the dashed line 711 inFIG. 8A.

The physical layout of FIG. 8A and the corresponding electrical circuitof FIG. 8B illustrate an interface by which operation of azero-heat-flux DTT measurement device with a thermal sensor calibrationcircuit can be controlled and monitored in a DTT measurement system.FIG. 9 illustrates a signal interface between a zero-heat-flux DTTmeasurement device according to FIGS. 7A and 7B, using the firstconstruction of FIG. 8A as an example. With reference to these figures,a DTT measurement system includes control mechanization 800, ameasurement device 700, and an interface 785 that transfers power,common, and data signals between the control mechanization and themeasurement device. The interface can be wireless, with transceiverslocated to send and receive signals. Preferably, the interface includesa cable 787 with a connector 789 releasably connected to the tab 708.The control mechanization 800 manages the provision of power and commonsignals on respective signal paths to the heater and provides for theseparation of the signals that share a common signal path, such as theThermistor2 (TH2) and SCL signals. A common reference voltage signal isprovided on a single signal path to the thermal sensors, and respectiveseparate return signal paths provide sensor data from the thermalsensors.

Presuming that the thermal sensor calibration circuit 770 includes anEEPROM, a separate signal path is provided for EEPROM ground, and thethermal sensor signal paths are shared with various pins of the EEPROMas per FIGS. 8A and 8B. This signal path configuration separates thedigital ground for the EEPROM from the DC ground (common) for theheater, for good reason. Presume that the EEPROM and the heater share anelectrical pad for ground. The cable 787 including its connectorcontacts has a certain amount of resistance. If the heater 726 ispowered up, the current through it has to return to the controlmechanization 800 through the ground (common) contact, which means therewill be some voltage developed on the measurement device side of thecontact equal to the resistance of that line multiplied by the currentthrough the heater 726. That voltage could be as high as 2 or 3 voltsdepending on the integrity of the contacts. If concurrently the supplyvoltage goes low on the EEPROM or even one of the logic lines goes lowbelow this aforementioned generated voltage, the EEPROM would bereversed biased which could damage the part. Separating the heater andEEPROM grounds eliminates all these possibilities for damage to theEEPROM. Accordingly, it is desirable to electrically isolate the heateraltogether from the other elements of the electrical circuit. Thus, asper FIG. 9, a first electrical pad (electrical pad 5, for example) ofthe plurality of electrical pads is connected only to a first terminalend of the heater trace, while a second electrical pad (electrical pad6, for example) of the plurality of electrical pads is connected only tothe second terminal end of the heater trace.

With reference to FIG. 8B, presume that the thermal sensors are NTCthermistors. In this case, the common signal on electrical pad 2 is heldat a constant voltage level to provide Vcc for the EEPROM and areference voltage for the thermistors. Control is switched via thethermistor/EEPROM switch circuit between reading the thermistors andclocking/reading/writing the EEPROM. Presuming again that the thermalsensors are NTC (negative temperature coefficient) thermistors, theEEPROM has stored in it one or more calibration coefficients for eachthermistor. When the device 700 is connected to the controlmechanization, the calibration coefficients are read from the EEPROMthrough the SDA port in response to a clock signal provided to the SCLport of the EEPROM. The following Table of Signals and ElectricalCharacteristics summarizes an exemplary construction of the interface785.

Table of Signals and Electrical Characteristics Element Signals andElectrical Characteristics Thermal sensors 740, 742 Common referencesignal is 3.3 volts DC. Outputs are analog. Heater 726 Total resistance6.5 to 7.0 ohms driven by a pulse width modulated waveform of 3.3 voltsDC. The power density of the peripheral portion 729 is 30%-60% higherthan that of the center portion 728. EEPROM 770 (Micron Ground is 0volts. Vcc is 3.3 volts DC. SCL and SDA pins see a Technology24AA01T-I/OT) low impedance source switched in parallel with thethermistor outputs.

In a best mode of practice, a temperature measurement device accordingto FIG. 8A has been fabricated using the materials and parts listed inthe following table. An electrical circuit with copper traces and padswas formed on a flexible substrate of polyimide film by a conventionalphoto-etching technique and thermal sensors were mounted using aconventional surface mount technique. The dimensions in the table arethicknesses, except that Ø signifies diameter. Of course, thesematerials and dimensions are only illustrative and in no way limit thescope of this specification. For example, the traces may be made whollyor partly with electrically conductive ink. In another example, thethermal sensors are preferably thermistors, but PN junctions,thermocouples, or resistance temperature detectors can also be used.

Table of Materials and Parts: II Representative Element Material/Partdimensions/characteristics Flexible substrate 701, 2 mil thickPolyethylene terephthalate Substrate 701: 0.05 mm thick heater 726,contacts, (PET) film with deposited and photo- and pads etched ½ oz.copper traces and pads and immersion silver-plated contacts. Thermalsensors 740, Negative Temperature Coefficient 10k thermistors in 0603package. 742 (NTC) thermistors, Part # R603-103F- 3435-C, RedfishSensors. Flexible insulating layers Closed cell polyethylene foam withInsulator 702: Ø40 x 3.0 mm thick 702, 709 skinned major surfaces coatedwith Insulator 709: Ø40 x 3.0 mm thick pressure sensitive adhesive (PSA)Stiffener 10 mil thick PET film Stiffener: 0.25 mm thick EEPROM 770Micron Technology 24AA01T-I/OT

According to the best mode, calibration coefficients for the thermistorsare obtained and stored in the EEPROM. The basis of obtaining accuratetemperature sensing from the negative temperature coefficientthermistors is through calibration. The resistance of each thermistordecreases in a generally logarithmic relationship as temperatureincreases. Two models exist which provide adequate precision to resultin ±0.05° C. temperature accuracy over a 70° C. span [Fraden, J., “Atwo-point calibration of negative temperature coefficient thermistors,”Rev Sci Instru 71(4):1901-1905]. The best known is the Steinhart andHart model:

T=[b ₀ +b ₁ ln R+b ²(ln R)²]⁻¹  Equation 1

which relates resistance, R, to temperature, T, as a function of threeconstants, b₀, b₁, and b₃. Calibration entails placing the DTTmeasurement device in three successively higher thermally controlledenvironments and recording the resistance at each condition. Theconstants may then be solved for using three simultaneous equations. Thethree resulting constants for each individual thermistor are thenrecorded on the EEPROM on the DTT measurement device.

A simplified model by Fraden, is of the form:

$\begin{matrix}{{\ln \; R} \cong {R_{0} + \frac{\beta_{0}\left\lbrack {1 + {\gamma \left( {T - T_{0}} \right)}} \right\rbrack}{T}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

where the terms, β₀, γ, R₀, and T₀ are constants for a given sensor. Thebeta and gamma terms are related by the form:

$\begin{matrix}{\gamma = {\frac{\left( {\frac{\beta_{1}}{\beta_{0}} - 1} \right)}{T_{1} - T_{0}}.}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

The value of gamma is a normalized slope of beta. This is a linearrelationship and gamma may be approximated as a constant for a giventhermistor type. Calibration using the model proposed by Fraden thusentails placing the DTT measurement device in only two successivelyhigher-temperature-controlled environments and recording the resistanceat each temperature. The R₀ and T₀ are one of those sets of measuredvalues. The four constants noted above for each individual thermistorare then recorded on the EEPROM on the DTT measurement device.

In a second construction of the measurement device 700, illustrated inFIG. 10, no slits are provided in the substrate 701, and so the heater726 includes continuous central and peripheral portions 728 and 729 withdifferent power densities. Six electrical pads having the sameconnections as shown in FIGS. 8A and 8B are provided on the tab 708.

In third and fourth constructions of the measurement device 700,illustrated in FIGS. 11 and 12, respectively, the heater trace includesthree traces: a first trace 810 that defines the central heater portion728, a second trace 811, surrounding the first trace 810, that definesthe peripheral heater portion 729, and a third trace 812 connected tothe first and second traces at a shared node 814. The third trace 812serves as a common connection between the first and second traces. Thisheater construction is thus constituted of independently-controlledcentral and peripheral heater portions that share a common lead.Alternatively, the construction can be considered as a heater with twoheater elements. The power densities of the central and peripheralportions can be uniform or nonuniform. If the power densities of the twoportions are uniform, the peripheral portion can be driven at a higherpower level than the central portion so as to provide the desired higherpower density. As per FIGS. 8B, 9, 11, and 12 the second heaterconstruction requires three separate pins (6, 7, and 5) for the first,second, and third traces. Thus, for a construction of the electricalcircuit that includes two independently-controlled heater portions thatshare a common lead, seven electrical pads are provided on the tab 708.As with the first heater construction, the heaters of the second heaterconstruction are entirely electrically isolated from the other elementsof the electrical circuit. In this regard, with reference to FIGS. 9 and11, the heater trace 726 includes three terminal ends and a firstelectrical pad (electrical pad 5, for example) of the plurality ofelectrical pads is connected only to a first terminal end of the heatertrace, a second electrical pad (electrical pad 6, for example) of theplurality of electrical pads is connected only to the second terminalend of the heater trace, and a third electrical pad (electrical pad 7,for example) of the plurality of electrical pads is connected only tothe third terminal end of the heater trace.

It is not necessary that the flexible substrate be configured with acircular central section, nor is it necessary that the annular heater begenerally circular. In ninth and tenth constructions of the measurementdevice 700, illustrated in FIGS. 13 and 14, respectively, the centralsubstrate sections have multilateral and oval (or elliptical) shapes, asdo the heaters. All of the constructions previously described can beadapted to these shapes as required by design, operational, ormanufacturing considerations.

Although principles of temperature measurement device construction andmanufacture have been described with reference to presently preferredembodiments, it should be understood that various modifications can bemade without departing from the spirit of the described principles.Accordingly, the principles are limited only by the following claims.

1. A zero-heat-flux temperature device with first and second flexiblesubstrate layers sandwiching a layer of thermally insulating material,comprising: a heater trace disposed on the first substrate layerdefining a heater surrounding a zone of the first substrate layer havingno heater trace, a first thermal sensor disposed in the zone, a thermalsensor calibration device, a second thermal sensor disposed on thesecond substrate layer, a plurality of connection pads, and a pluralityof conductive traces connecting the heater trace and the first andsecond thermal sensors with the plurality of connection pads.
 2. Thezero-heat-flux temperature device of claim 1, in which the thermalsensor calibration device includes a programmable memory.
 3. Thezero-heat-flux temperature device of claim 1, in which the flexiblesubstrate includes a center section, a tab extending outwardly from theperiphery of the center section, and a tail extending outwardly from theperiphery of the center section, the plurality of connection pads isdisposed on the tab, and the center section and the tail are foldedaround the layer of thermal insulating material such that the centersection constitutes the first substrate layer and the tail constitutesthe second substrate layer.
 4. The zero-heat-flux temperature device ofclaim 3, in, which the thermal sensor calibration device is disposed ona surface portion of the substrate extending partially over the tab andthe center section.
 5. The zero-heat-flux temperature device of claim 4,in which the thermal sensor calibration device includes a programmablememory.
 6. The zero-heat-flux temperature device of claim 3, in whichthe thermal sensor calibration device is disposed between the heater andthe plurality of connection pads.
 7. The zero-heat-flux temperaturedevice of claim 6, in which the plurality of conductive traces includesconductive traces connecting the thermal sensor calibration device withconnection pads of the plurality of connection pads.
 8. Thezero-heat-flux temperature device of claim 3, in which the tab includesopposing notches to receive and retain retainers of a cable connector.9. The zero-heat-flux temperature device of claim 8, in which theannular heater trace includes two terminal ends and a first connectionpad of the plurality of connection pads is connected only to a firstterminal end of the heater trace and a second connection pad of theplurality of connection pads is connected only to the second terminalend of the heater trace.
 10. The zero-heat-flux temperature device ofclaim 8, in which the annular heater trace includes three terminal endsand a first connection pad of the plurality of connection pads isconnected only to a first terminal end of the heater trace, a secondconnection pad of the plurality of connection pads is connected only tothe second terminal end of the heater trace, and a third connection padof the plurality of connection pads is connected only to the thirdterminal end of the heater trace.
 11. A temperature device, comprising:a flexible substrate; and, an electrical circuit on the flexiblesubstrate, the electrical circuit including an annular heater tracesurrounding a zone of the substrate, a first thermistor disposed in thezone, a second thermistor disposed outside of the annular heater trace,a thermistor calibration device, a plurality of connection pads, and aplurality of conductive traces connecting the first and secondthermistors, the thermistor calibration device, and the heater tracewith the plurality of connection pads.
 12. The temperature device ofclaim 11, in which the plurality of connection pads includes six orseven connection pads.
 13. The temperature device of claim 11, in whichthe thermistor calibration device is a programmable memory devicestoring thermistor calibration coefficients.
 14. The temperature deviceof claim 11, in which the thermistor calibration device is disposed on asurface portion of the flexible substrate between the heater trace andthe connection pads.
 15. The temperature device of claim 14, in whichthe thermistor calibration device is a programmable memory devicestoring thermistor calibration coefficients.
 16. The temperature deviceof claim 15, in which the plurality of connection pads includes six orseven connection pads.